The present invention relates to the field of data recording and playback. More particularly, it relates to improvements in radial access apparatus for controlling the radial position of a read/write head relative to a data storage disk, whereby the head may be moved from one data track to another to playback or record data on the disk.
Linear motors of the moving coil variety are used extensively in applications requiring linear force-current characteristics and fast response. An example of such applications is the radial access apparatus used in a conventional disk drive for moving a read/write head between the various data tracks on an optical, magneto-optical, or magnetic data storage disk. Such moving coil radial access mechanisms are disclosed, for example, in U.S. Pat. Nos. 3,521,092; 4,439,699; and 4,603,270.
FIGS. 1 and 1A illustrate the essential features of a typical moving coil linear motor M. Such device comprises a coil C wound upon one end of a tubular bobbin 10 which supports a load, shown as the read/write head 12 of a data recording system. The coil is suspended in a radial magnetic field produced by a tubular permanent magnet 14. The tubular magnet is concentrically arranged with respect to the central post 16 of a cylindrical housing 18 made of steel or other magnetically permeable material. The outer surface of magnet 14 is press fit against the inside surface 20a of a peripheral cylindrical sleeve 20 of housing 18, and a base portion 22 of the housing provides a return path for the magnetic flux F, shown in phantom line. Depending upon the direction of current flow within the coil, the coil moves in a linear direction, as indicated by the arrows A, in an air gap G defined by the inside surface of the magnetic member 14 and the exterior surface of post 16. The coil experiences a force directly proportional to the magnetic flux density B provided by magnetic member 14, and the current I in the coil, the force on the coil being normal to both B and I. As the coil moves linearly in the magnetic field, the read/write head 12 moves radially with respect to a data storage disk D, such as an optical, magneto-optical, or magnetic disk. A motor M operates through a spindle S to rotate the disk D about its central axis, while the linear motor operates to move the read/write head 12 along the disk radius to access one of the concentrically arranged data tracks on the disk.
Incremental and high speed motion control of the read/write head requires rapid changes in the magnitude and direction of the force applied to coil C. Since the flux density B is fixed, rapid changes in force imply rapid changes in current. In order to achieve a short current rise time, it is common practice to enclose the central steel post 16 with a non-magnetic, electrically conductive sleeve 24, commonly referred to as a "shorted turn." Currents induced in the shorted turn by the change in flux produced during movement of the coil act to reduce the magnetic flux diffusing into the high permeability post 16. As a result, the back emf seen at the coil terminals is reduced, thereby reducing the current rise time.
While the sleeve-shaped shorted turn shown in FIG. 1 offers the advantage of reducing the current rise time, it is disadvantageous from the standpoint that it also reduces the available flux density in the air gap. Thus, while a shorted turn of this type will increase the rate of rise of force on the coil, the final magnitude of such force will be smaller, since the magnetic field is less.